Republicans are eager to use former Sen. Russ Feingold's signature issue against him in 2016, as the Democrat wages a comeback bid in a Senate contest key to his party's quest for the majority.

After losing re-election to a fourth term in 2010, Feingold, who had championed limits on spending in federal elections, launched the Progressives United PAC, which the Milwaukee Journal Sentinel reported spent a large percentage of the $7.1 million it raised on overhead and staff salaries. Republicans are already attempting to use that news to paint Feingold as a hypocrite.

"This kind of hypocrisy is going to disappoint a lot of Wisconsinites — and it should," Feingold's opponent, Sen. Ron Johnson, R-Wis., said in a statement to CQ Roll Call. "Sen. Feingold has become just like every other career politician in Washington, saying one thing while doing another."

Feingold is the co-author of the 2002 McCain-Feingold campaign finance law that set limits on federal election spending. In 2010, after the Citizens United Supreme Court case allowed corporations and unions to spend unlimited funds in elections, Feingold refused to allow those groups to support his re-election bid against Johnson. Feingold went on to lose re-election to Johnson by a 5-point margin in the 2010 tea party wave.

Brad Todd, a GOP ad maker whose firm worked on Johnson's 2010 contest, said the Feingold PAC's spending "runs completely counter" to what Feingold has championed.

"This goes right to the core of Russ Feingold's highest priorities," he said. "If his highest priority turns out to have been hypocrisy, what else can you trust him on?"

Democrats, however, deny there were any financial misdoings at Feingold's PAC. Wisconsin Democratic operatives said Progressives United is not a super PAC — which can spend unlimited resources for candidates. Instead, they say it was an advocacy group that sought to push for campaign finance reform, and in the process gave funds to candidates who also held that same goal.

"The accusation that there’s hypocrisy on super PACs is wrong, because there is no Russ Feingold super PAC. He’s always said those shouldn’t exist," said a Wisconsin Democratic operative close to Feingold's campaign. "They're trying to get what they want here, when it's not the truth."

Democrats add that Johnson's attack on Feingold's PAC is an attempt to create cover to avoid accepting Feingold's "Badger Pledge" that would keep super PACs out of their contest.

Feingold issued the pledge — modeled after the "people's pledge" Elizabeth Warren and Scott P. Brown agreed to in their 2012 Massachusetts Senate race — three days before the Journal Sentinel published its story on Feingold's Progressives United PAC.

And Democrats have tried to make that the issue.

“It’s been a week since Russ challenged Ron Johnson to keep outside money out of the race, and Johnson is transparently pretending he has some serious thinking to do when the truth is he’s either for outside money or he’s against it,” said Justin Barasky, communications director at the Democratic Senatorial Campaign Committee.

While the debate over campaign finance has gotten louder since Citizens United, the issue has yet to sway an election. And explaining it is too complicated for a typical 30-second ad. And in 2016, the presidential contest is likely to have the most impact on the race.

The last time Wisconsin voted for a Republican presidential nominee was in 1984, when Ronald Reagan swept almost every state to win a historic victory. President Barack Obama carried it by a 7-point margin in 2012, and a 14-point margin four years earlier.

The state's presidential turnout could make Johnson's road to re-election steeper, as he could need to sway a large portion of the electorate that's voting for the Democratic presidential nominee to also support his bid.

Republicans are counting on their allegations against Feingold making the road a little less steep.

“I wouldn’t necessarily say this is the ‘Jurassic World’ of political bombshells, but I think it will have a significant impact with at least some part of the electorate,” Scott Becher, a longtime Wisconsin GOP operative, said of the Journal Sentinel's reporting.